The desulfurization and denitrification of flue gases from large furnace plant today occupies a very important position for the protection of the environment.
Besides catalytic dry processes and some wet processes, some of which operate simultaneously and some of which operate selectively, in Japan there has been developed in recent years a physical process in which the conversion of SO.sub.2 and NO.sub.x is carried out by irradiation with accelerated electrons in the presence of ammonia. There are thereby formed ammonium sulphate and ammonium nitrate which are separated by means of air filter plants. In the case of this process, which is described, for example, In Radiat. Phys. Chem., 18, Nos. 1-2, pp. 389-398/1981, the flue gases are irradiated with two oppositely positioned sources of electron beams of relatively high acceleration voltage (750 keV) in a round flowthrough reactor with simultaneous mixing up.
The electrons accelerated in a vacuum pass out through a first metal foil to the atmosphere (normally air), penetrate through this air gap and pass through a second metal foil into the reaction vessel. The first and second metal foils absorb a considerable part of the electron beam energy. For this reason, they must be cooled by a current of compressed air which is blown through between the metal foils. The metal foils consist, for example, of titanium.
The reactor is of round cross-section; it is only slightly flattened at the places of the electron entry. The reactor vessel is so constructed that the whole of the electron beam is absorbed by the flue gas and as little as possible radiation losses occur on the walls of the reactor.
In order to homogenize the irradiation in the reactor, the gas is additionally circulated by an impeller during the electron bombardment.
The two sources of electron beams are the important part of this process. Thus, the economy as well as the industrial utility depends upon the optimization of these two sources of electron beams.
Due to the use of the sources of electron beams with the relatively high acceleration voltage (750 keV), the following disadvantages result:
(1) Due to the high acceleration voltage of 750 keV, the irradiation can only be economically screened with concrete, which results in an immobility. It is not possible to work in the direct surroundings of the irradiator during the operation.
(2) Due to the fact that a metal foil is provided for the electron emergence from the accelerator and a second metal foil is provided for the electron entry into the reactor vessel, much energy is absorbed by the two metal foils. The compressed air current necessary for the window cooling is partly oxidized to ozone by the electron irradiation. This ozone attacks the apparatus and is a factor which has an adverse effect on the environment. If an inert gas, for example nitrogen, is used for cooling the two foils, then this is very expensive due to the large amounts of cooling gas necessary.
(3) There are no optimum conditions in a round reactor because of the absorption and scattering conditions of the electrons. The absorption of the electrons takes place in the form of a flattened bell-shaped curve (see the ionization curves for one-sided irradiation; FIG. 3 of the accompanying drawings). The scattering of the electrons in the reactor vessel in the case of irradiation with a source of electron beams gives a pear-shaped distribution seen from the electron entry into the reactor.
(4) The electron beam dosage necessary for the irradiation of flue gases lies in the order of magnitude of 2 Mrd. This dosage capacity of the sources of electron beams lies at about 40 Mrd/sec., i.e. the gas exchange in the electron irradiation region must take place at least 20 times per second in order to achieve the most economic irradiation. The smaller is the irradiation field constructed in the case of the same electron beam capacity, then the more favorable are the irradiation conditions.
Therefore, it is an object of the present invention to provide a process and a device which no longer display the above-described disadvantages.
Thus, according to the present invention, there is provided a process for the desulfurization and denitration of flue gases by electron irradiation of the flue gases to which ammonia has been added prior to the irradiation, wherein for the electrons there are used low-energy sources of electron beams with an acceleration voltage for the electrons of 250 keV, only one of which has an electron exit window, the support lattice of which is formed from several double comblike bridges elements disconnectably connected with the windowframe, each of which has a main bridge which is provided with a bore as cooling pipe.
The present invention also provides a device for the desulfurization and denitration of flue gases by electron beam irradiation of the flue gases to which ammonia has been added prior to the irradiation, consisting essentially of a reaction canal and two sources of electron beams, wherein low energy sources of electron beams are used with an acceleration voltage for the electrons of 250 keV, of which only one has an electron exit window, the support lattice of which is formed from several double comb-like bridge elements disconnectably connected with the window frame, each of which has a main bridge which is provided with a bore as cooling pipe, the electrons emerging from the source of electron beams through the electron window passing directly into the reaction canal.